U.S. patent number 5,613,452 [Application Number 08/419,292] was granted by the patent office on 1997-03-25 for method and apparatus for soil remediation with superheated steam thermal desorption and recycle.
This patent grant is currently assigned to American Color and Chemical Corporation. Invention is credited to George M. Goyak, Primo Marchesi.
United States Patent |
5,613,452 |
Marchesi , et al. |
March 25, 1997 |
Method and apparatus for soil remediation with superheated steam
thermal desorption and recycle
Abstract
A method and apparatus are provided for remediating contaminated
soil having volatilizable organic pollutants in a heated treatment
zone, preferably a rotary drum, in gas/solids contact with
superheated steam. Treatment gases from the treatment zone are
repressurized and reheated for reuse in the treatment zone.
Superheated steam is maintained in a closed loop and is maintained
at superheated conditions. Vaporized organic pollutants and
superheated steam are removed from the closed loop of superheated
steam for cooling and condensation to condense water and condense
liquid volatilizable organic pollutants and uncondensed organic
pollutant vapors.
Inventors: |
Marchesi; Primo (Lock Haven,
PA), Goyak; George M. (Murrysville, PA) |
Assignee: |
American Color and Chemical
Corporation (Lock Haven, PA)
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Family
ID: |
21997762 |
Appl.
No.: |
08/419,292 |
Filed: |
March 30, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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55433 |
Apr 29, 1993 |
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Current U.S.
Class: |
110/215; 110/236;
110/346; 110/348; 405/128.85 |
Current CPC
Class: |
B09B
3/0091 (20130101); B09C 1/06 (20130101) |
Current International
Class: |
B09C
1/00 (20060101); B09C 1/06 (20060101); F23J
015/00 (); B09B 003/00 () |
Field of
Search: |
;110/215,216,246,346,348,236,204 ;34/92,15,79 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"In Situ Steam/Hot-Air Soil Stripping", Toxic Waste (USA) Inc., EPA
Site Technology Evaluation Demonstration Bulletin,
EPA/540/M5-90/003, Feb., 1990. .
"Batch Steam Distillation and Metal Extraction" It Corporation, EPA
Site Technology Profile, pp. 224-225, Nov. 1991; EPA Vendor
Information System for Innovative Treatment Technologies (Visitt),
pp. 1-22, Sep. 27, 1991. .
"Thermal Desorption by Steam Stripping/Solid Waste Desorption",
Texarome Inc. EPA Site Technology Profile, pp. 152-153, Nov. 1991;
EPA Visitt, pp. 1-14 Aug. 19, 1991; and Related Excerpt, pp. 1-3.
.
"Here's How it Works" Clean Soil Steam Remediation Technology,
Soils p. 42, May 1992..
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Primary Examiner: Bennett; Henry A.
Assistant Examiner: Tinker; Susanne C.
Attorney, Agent or Firm: Bartlett; Ernestine C.
Parent Case Text
This is a continuation of application Ser. No. 08/055,433, filed
Apr. 29, 1993, abandoned.
Claims
We claim:
1. A method for reducing the volatilizable organic pollutant
content of soil which is contaminated with at least one
volatilizable organic pollutant, comprising:
delivering contaminated soil to a treatment zone and passing said
contaminated soil through said treatment zone in vapor/solids
contact with treatment vapors consisting essentially of
predominantly superheated steam and devolatilized organic pollutant
whereby moisture from said contaminated soil is converted to steam
and a substantial portion of said volatile organic pollutant is
volatilized;
maintaining said treatment zone at a pre-selected temperature to
cause moisture from said soil to convert to steam and a substantial
portion of said organic pollutants to volatilize,
recovering from said treatment zone a relatively dried soil having
substantially lower pollutant content than said contaminated
soil;
recovering as an exit gas stream at an exit temperature and an exit
pressure the treatment vapors from said treatment zone, said exit
gas stream comprising predominantly superheated steam and a minor
portion of volatilized organic pollutants; and
recirculating a major portion of said recovered exit gas steam at a
pressure and temperature greater than said exit pressure and said
exit temperature and consisting essentially of predominantly
superheated steam and a minor portion of volatilized organic
pollutant to said treatment zone as the treatment vapors
therein.
2. The method of claim 1 including the additional step of heating
said treatment zone and its contents independently of said
superheated steam.
3. The method of claim 1 including the additional steps of
recovering a minor portion of said exit stream and cooling said
minor portion of said exit gas stream to condense superheated steam
and to condense substantially all of the condensible volatilized
organic pollutants therein to a liquid state.
4. The method of claim 3 wherein the mass flow ratio of said exit
gas stream to the mass flow of said minor portion of said exit gas
stream exceeds between 5:1 to 30:1.
5. The method of claim 1 wherein said exit pressure is less than 10
psig.
6. The method of claim 1 wherein said exit pressure is less than
one atmosphere.
7. The method of claim 1 wherein said exit gas stream contains
gas-borne particles and said exit gas stream is passed through a
solids-gas separator to remove a substantial quantity of said
gas-borne particles from said exit gas stream.
8. The method of claim 7 wherein said solid-gas separator is heated
to maintain an elevated temperature therein above the temperature
of gas and solids in said separator.
9. The method of claim 1 wherein said treatment zone is a rotary
drum.
10. The method of claim 9 wherein the rotary drum is heated to
provide a portion of the thermal energy required to heat said
contaminated soil and to maintain said rotary drum at a
pre-selected temperature profile.
11. A method for reducing the volatilizable organic pollutant
content of particulate soil which is contaminated with at least one
volatilizable organic pollutant, comprising:
delivering contaminated particulate soil to a treatment zone and
passing said contaminated soil through said treatment zone in
vapor/solids contact with treatment vapors consisting essentially
of predominantly superheated steam whereby moisture from said
contaminated particulate soil is converted to steam and a
substantial portion of said volatile organic pollutant is
volatilized;
heating said treatment zone to maintain a pre-selected temperature
therein;
recovering from said treatment zone a dried, particulate soil
having substantially lower pollutant content than said contaminated
particulate soil;
recovering as an exit gas stream at an exit temperature and an exit
pressure the treatment vapors from said treatment zone, said exit
gas stream comprising predominantly superheated steam and a minor
portion of volatilized organic pollutants;
removing a minor portion of said exit gas stream;
increasing the pressure of the remaining exit gas stream to a
pressure greater than said exit pressure and heating the resulting
pressurized gas stream to an elevated temperature above said exit
temperature;
recirculating the resulting gas stream at a pressure and
temperature greater than said exit pressure and said exit
temperature and comprising predominantly superheated steam and a
minor portion of volatilized organic pollutant to said treatment
zone as the treatment vapors therein, whereby said gas stream and
said treatment vapors consist essentially of predominantly
superheated steam; and
cooling said removed minor portion of said exit gas stream to
condense superheated steam and to condense substantially all of the
condensible volatilized organic pollutants therein to a liquid
state.
12. The method of claim 11 including the additional step of heating
said treatment zone and its contents independently of said
superheated steam.
13. The method of claim 11 wherein the mass flow ratio of said exit
gas stream to the mass flow of said minor portion of said exit gas
stream exceeds between 5:1 to 30:1.
14. The method of claim 11 wherein said exit pressure is less than
10 psig.
15. The method of claim 11 wherein said exit pressure is less than
one atmosphere.
16. The method of claim 11 wherein said exit gas stream contains
gas-borne particles and said exit gas stream is passed through a
solids-gas separator to remove a substantial quantity of said
gas-borne particles from said exit gas stream.
17. The method of claim 11 wherein said treatment zone is a rotary
drum.
18. The method of claim 11 wherein the rotary drum is heated to
provide at least a major portion of the thermal energy required to
heat said contaminated soil and to maintain said rotary drum at a
pre-selected temperature profile.
19. Apparatus for treating solid materials which are contaminated
with at least one organic pollutant in order to reduce the content
of said pollutant, including:
(a) a treatment zone having a treatment chamber, means for
introducing contaminated solid materials into said chamber, means
for recovering dried particulate solid materials from said chamber
having a substantially reduced organic pollutant content, means for
introducing treatment vapors consisting essentially of superheated
steam into said treatment chamber, means for establishing effective
vapor/solids contact in said chamber between said treatment vapors
and said solid materials, and means for recovering an exit gas
stream from said chamber comprising predominantly superheated steam
and a minor portion of vaporized moisture from said solid materials
and a minor portion of volatilized organic pollutant;
(b) a gas-pressurizing means; a gas-heating means; first conduit
means extending from said chamber means to said gas pressurizing
means; second-conduit means extending from said gas-pressurizing
means to said gas-heating means; and third conduit means extending
from said gas-heating means to said treatment chamber, providing a
closed loop for circulation of treatment vapors from said treatment
chamber through said first conduit, through said gas-pressurizing
means, through said second conduit means, through said gas-heating
means, and through said third conduit means to said treatment
chamber; and
(c) condenser means for cooling a gas stream; fourth conduit means
connecting said condenser means and said first conduit means or
said second conduit means for delivering a gas stream from said
first conduit means or said second conduit means to said condenser
means.
20. The apparatus of claim 19 including heating means for heating
said treatment chamber independently of said superheated steam.
21. The apparatus of claim 19 wherein said treatment zone is a
rotary drum.
22. The apparatus of claim 19 including gas/solids separating means
in said first conduit means.
23. The apparatus of claim 22 including heating means for
maintaining a pre-selected temperature within said gas solids
separating means.
Description
FIELD OF THE INVENTION
This invention relates to a method and apparatus for the
remediation of contaminated soil by removing volatilizable organic
pollutants which contaminate the soil to permit reuse of the soil
and recovery of volatilizable organic pollutants. More
particularly, the invention relates to a method and apparatus for
recovering volatilizable organic pollutants from contaminated soil
by means of treatment with superheated steam maintained in a
closed, circulating loop.
BACKGROUND OF THE INVENTION
Soil remediation is well established as a procedure for complying
with environmental clean-up requirements. Continued accumulations
of volatilizable organic pollutants in the soil around chemical
plants, petroleum plants, manufacturing plants, gasoline filling
stations, and agricultural chemical deposits (e.g., pesticides,
herbicides, fungicides, etc.) may be considered as a threat to
surface water and ground water or a threat to one or more other
circumstance which is regulated by environmental laws and rules.
Where contaminated soil is objectionable, there are numerous
regulations to be considered.
The present invention is concerned with contaminated soil
remediation where the volatilizable organic content of the soil
exceeds the allowable regulatory maxima but is preferably less than
about five weight percent of the soil and more particularly less
than about two weight percent of the soil. Examples of suitable
soils include solids such as topsoil, river sediments, bedrock,
alluvium, and particulate fill materials such as cinders, gravel
and slag.
Several procedures for remediating contaminated soils are shown in
U.S. Pat. Nos. 4,738,206; 4,974,528; 5,072,674; 5,103,578;
5,121,699; 5,142,998; 5,152,233; 5,187,131; (heat supply for
devolatilization by heated flight conveyor) U.S. Pat. Nos.
4,738,206; 5,072,674; 5,142,998; (indirect heating means, e.g.
electric heaters or heat exchange fluids) U.S. Pat. No. 5,103,578;
and (fuel oil or fuel gas combustion) U.S. Pat. Nos. 4,974,528;
5,121,699; 5,152,233.
None of the prior art processes employ superheated steam in
gas/solids contact with the contaminated soil in a system wherein
superheated steam and the volatilized and/or volatilizable organic
pollutants recirculate with the system at temperatures which
maintain the superheated system in a superheated state, i.e.,
wherein the steam is maintained above its saturation temperature at
all times.
SUMMARY OF THE INVENTION
According to the present invention, contaminated earth solids are
introduced into an enclosed treatment zone, preferably an
appropriately sealed rotating drum, which is maintained at an
elevated temperature which promotes volatilization of any
volatilizable organic pollutants from the contaminated soil.
Appropriate soil temperatures are 250.degree. F. to 1000.degree. F.
and preferably 300.degree. F. to 700.degree. F. depending on the
particular pollutants present in the soil and the particular
contaminated soil. The contaminated soil is introduced into the
treatment zone at ambient temperature and is heated within the
treatment zone to a pre-selected discharge temperature after which
a remediated soil having a residual organic pollutant content
acceptable within applicable laws and regulations is obtained.
The soil preferably is heated to a pre-selected temperature and
maintained at the pre-selected temperature within the rotary drum
for a residence time sufficient to achieve the desired pollutant
volatilization. Extended residence time with extended retention of
the contaminated soil in the treatment zone may achieve the desired
pollutant volatilization at a lower soil temperature. The
pre-selected temperature and pre-selected residence time will be
determined by the specific contaminated soil, the nature of the
organic pollutants and the maximum residual contamination under
applicable regulations or other considerations.
A stream of treatment gas passes through the treatment zone in
gas/solids contact with the contaminated soil. The treatment gas
consists of superheated steam and volatilized organic pollutants
which have been removed from contaminated soils in the prior
operation of the process. The treatment gas is withdrawn from the
treatment zone at an exit pressure and at an exit temperature which
is sufficient to maintain the steam in a superheated state. A major
portion of the treatment gas is pressurized, reheated and returned
to the treatment zone as the treatment gas. A portion of the exit
gas, consisting of superheated steam and volatilized, volatilizable
organic pollutants, is separated from the recirculating treatment
gas and is cooled to condense the superheated steam and most of the
vaporized organic pollutants. Any non-condensed organic pollutants
are recovered as a gas stream; the condensed organic pollutants are
recovered as a liquid stream; the condensed superheated steam is
recovered as water.
The hot decontaminated soil is recovered from the treatment zone
and is recycled to the environment. In a preferred mode, the
decontaminated soil is restored to the location from whence it
originated, i.e., the plot of land requiring remediation.
Alternatively the decontaminated soil may be dispersed in other
areas, for example, the decontaminated soil may qualify as a cover
for municipal landfills. The water recovered from the process may
be used to cool and moisten the decontaminated soil prior to reuse
of the decontaminated soil.
In one embodiment, the exit gas from the treatment zone may be
passed through a gas/solids separator such as a cyclone, a
baghouse, an impingement knock-out box, etc. to remove gas-borne
particles which may be objectionable in the subsequent gas
treatment. Preferably such a gas/solids separator will be heated to
a temperature above the temperature of the gas stream passing
through the gas/solids separator to preclude deposition of volatile
organic materials from the gas stream. Solids recovered from the
gas/solids separator can be returned to the treatment zone or can
be separately disposed of in any suitable or desirable procedure,
e.g., appropriate landfilling.
The present invention and apparatus permit relatively easy
separation of the recovered volatilized organic pollutants from the
condensed superheated steam. Depending upon the nature of the
contaminating organic pollutants, appropriate recovery for recycle
may be feasible as in the case of, for example, recovered benzene,
toluene, xylene; aliphatic and cyclic petroleum products, etc.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE, FIG. 1, is a schematic illustration of the
apparatus assembled according to the invention for carrying out the
novel methods.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Examples of contaminated soil that may be used for the purposes of
this invention include earth materials (e.g., topsoil, river
sediments, bedrock, alluvium) or particulate fill material (e.g.,
cinders, gravel, slag) and others which contain objectionable
quantities of volatilizable organic pollutants, i.e., quantities
exceeding the limits imposed by environmental laws and regulations.
Preferably, organic pollutant content should not exceed about 5% by
weight of the earth solids and most preferably should not exceed
about 2% by weight of the earth solids in order to benefit from the
present invention although soils with higher pollutant content may
be satisfactorily treated.
Volatilizable organic materials are those organic materials which
can enter into the vapor phase at the temperatures anticipated in
the treatment zone. The volatilizable organic pollutants thus
include:
normally volatile materials which may be dissolved or absorbed in
the soil (e.g., acetone, paint thinners, etc.);
normally liquid organic materials which may be absorbed or
otherwise contained in the soil (e.g., benzene, toluene, xylene,
gasoline, fuel oil, lubricating oil, etc.);
materials which may be solid or semi-solid at ambient temperature
but which can be volatilized at elevated temperatures (e.g., heavy
oil, grease, light asphalt, tars, etc.);
agricultural chemicals such as pesticides, herbicides,
rodenticides, fertilizers, etc.; and
halogenated organic materials such as halogenated aromatics (PCBs)
and halogenated aliphatics, etc.
For convenience, the volatilizable organic materials are sometimes
referred to herein as "pollutants" to indicate that they are not
naturally occurring ingredients of the soil.
Superheated steam is steam which is maintained at a temperature
above its saturation temperature with liquid water. Superheated
steam can exist at sub-atmospheric pressure, at atmospheric
pressure and at super-atmospheric pressures. Superheated steam at
all times contains the latent heat of vaporization and at least
some sensible heat or enthalpy.
Referring to FIG. 1, the principal elements of the apparatus
are:
10 a supply site for contaminated earth solids;
11 a rotary drum which is a treatment zone for gas/solids
contact;
12 pump or blower means for increasing the pressure of
recirculating treatment gas;
13 a steam superheater for increasing the temperature of
recirculating treatment gas;
14 a condenser for recondensing a portion of the recirculating
treatment gas;
17 a liquid collector shown as a decanter for separate recovery of
non-condensed organic gases, condensed organic liquids and water;
and
18 a collection site for decontaminated soil.
The Treatment Zone
The treatment zone preferably is a rotary drum. Typically rotary
drums have circumferential supporting bands 20 (two are shown)
which support the drum in a position with the central lengthwise
axis tilted from the horizontal to facilitate movement of solids
through the rotary drum. Another band 19 may be provided with
perimeter teeth 21 which engage a toothed gear driving gear 22
driven by an appropriate drive means 23, usually a motor and speed
reducer, to turn the rotary drum about its central lengthwise axis.
The other bands 20 are supported on trunnion rolls 24 and thrust
rolls (not shown). The rotary drum 11 rotates in accordance with
the manufacturer's specification for the desired solids throughput.
The rotary drum customarily is equipped with agitating flights
extended radially inwardly from the inner cylindrical wall of the
drum. The flights may have angled surfaces to facilitate lifting
and showering of earth solids as they move through the rotary drum
from left to right in FIG. 1. The rotary drum 11 is a sealed drum
which has appropriate feeding means for receiving contaminated soil
from the supply site 10 through a conduit 25. An auger feeder is a
preferred feeding means for introducing contaminated soil into the
rotary drum 11. Double-valved lock-hoppers or star valves may be
used. Appropriate means may be provided for metering the flow of
contaminated soil. The rotary drum 11 also has sealed withdrawal
means for removing decontaminated soil from the rotary drum 11
through a conduit 26 to the collection site 18 for decontaminated
soil. The withdrawal means preferably is a double-valved
lock-hopper or star valve.
The rotary drum 11 can be any of several that may be obtained from
several manufacturers. One embodiment is a steel cylinder having
thermal insulation around the outer cylindrical wall and having
inwardly directed radial flights to agitate and advance the soil
moving through the drum.
In the preferred embodiment of the invention, all of the heat
requirements for the process are supplied by the sensible heat of
the recirculating superheated steam. In other embodiments, the
superheated steam may supply a portion of the heat requirements for
the process, with another portion of the heat requirements being
supplied by means of a heated rotary drum 11 which may be a heated
concentric rotary drum having two concentric steel cylinders in
which heating gases are burned in the annulus between the two
cylinders and the contaminated soil is delivered into the central
cylinder. Alternatively, the rotary drum may be heated
electrically, etc.
The rotary drums as described are readily available. The length and
diameter of a rotary drum determines the internal volume and hence
the throughput of the drum. A typical drum might be 5 feet in
diameter and 30 feet long for processing 1 to 5 tons per hour of
contaminated soil. The drum may range from about 3 feet to about 8
feet in diameter and have a length from 10 to 40 feet. In general,
about 10% of the volume of a rotary drum comprises the solid
materials undergoing treatment within the drum.
The rotary drum usually will exhibit a temperature profile with the
highest temperature adjacent to the solids discharge end of the
drum and the lowest temperature adjacent to the solids inlet of the
drum.
The Treatment Gas
The treatment gas consists of superheated steam and volatilized
organic pollutants which have been removed from previously treated
contaminated soil in the process. The treatment gas passes through
a closed loop consisting of:
the rotary drum 11; a conduit means 27 which leads from the rotary
drum 11 to the blower means 12; a conduit means 28 which leads to
the steam superheater 13; and a conduit means 29 which leads back
to the rotary drum 11.
Treatment gas enters into the rotary drum 11 through the conduit
means 29 at an elevated temperature, i.e., elevated above the
operating temperature within the rotary drum 11. The hot treatment
gas provides the heat of vaporization for the volatilizable organic
pollutants in the contaminated soil. The treatment gas also
provides at least some of the heat necessary for the heat of
vaporization of the moisture which is inherent in the contaminated
soil. Thus the treatment gas generates additional superheated steam
from the soil moisture. The treatment gas, the newly created
superheated steam and the volatilized, volatilizable organic
pollutants exit from the rotary drum 11 at an exit temperature and
an exit pressure and is delivered through conduit means 27 to the
pump (blower) means 12 where the pressure of the gases is increased
above the exit pressure. The exit pressure can be sub-atmospheric,
atmospheric or super-atmospheric. Typically the exit pressure is
from 0 to 5 psig. The exit temperature is sufficient to maintain
the superheated steam in a superheated state at the exit pressure,
and typically is 220.degree. F. to 500.degree. F. The blower means
12 increases the treatment gas pressure sufficiently to drive the
treatment gas through the superheater 13 and the rotary drum 11.
Typically the blower/pump means 12 raises the treatment gas
pressure to 2 to 15 psig, i.e., several psig above the exit
pressure. Relatively low pressures are preferred to avoid any
pressure-vessel piping requirements, i.e., the need to design,
build, test and maintain the system in accordance with industry
codes for pressure-vessels. Centrifugal compressors or positive
displacement blowers are preferred as the pump means. Optionally,
the process may be operated under subatmospheric pressure, e.g., at
14.7 to 13.8 psia. Subatmospheric pressure operation reduces any
tendency of the system to leak organic pollutants into the
environment. If such subatmospheric operation is employed, then the
inner portion of the exit gas stream will pass the pump means 12
and conduits 28, 31, 32 to the condenser 14; the conduit 30 will be
closed. During subatmospheric operation, the system must remain
relatively free of leaks which might allow air to enter the
recirculating superheated steam loop.
The Steam Superheater
The steam superheater 13 is a heat transfer device such as an
electrical core heater or a gas or fuel oil fired heater.
Electrical heating is preferred for precise control of the
process.
The pressurized treatment gases are delivered from the pump means
12 through the conduit 28 to the steam superheater where the
temperature of the treatment gases is increased above the exit
temperature. Typically the temperature of the treatment gases
entering the conduit 29 from the steam superheater 13 will be
500.degree. F. to 1100.degree. F. A product stream is withdrawn
from the treatment gas loop through conduits 30 or 31.
It should be observed from the preferred embodiment illustrated in
FIG. 1 that the volume of the rotary drum 11 greatly exceeds the
total volume of the conduit means 27, 28, 29 and the pump means 12
and superheater 13. Thus in any instant, the overwhelming majority
of the treatment gas is in the rotary drum 11 and a minor portion
of the recirculating treatment gas is in the remainder of the
closed loop consisting of the conduit means 27, 28, 29, the pump
means 12, and the superheater 13.
The Organic Pollutant Collection System
A portion of the recirculating treatment gas from the conduit 27 or
from the conduit 28 is recovered through conduits 30, 31
respectively and delivered through a conduit 32 to the condenser
14. The material passing into the condenser 14 corresponds to the
moisture and organic pollutant content of the contaminated soil.
The condenser is cooled by means of coolant fluids which are
delivered through a conduit 33, and withdrawn through a conduit
35.
If adequate cooling water is available, cool water will enter
conduit 33 and heated water will be withdrawn through conduit 35.
If cooling water is not available, a chiller (not shown) may
provide chilled coolant fluid through conduit 33. Heated coolant
fluid is withdrawn through conduit 35 and recovered for reuse as
coolant.
Substantially all of the superheater steam is cooled and condensed
in the condenser 14 along with substantially all of the condensible
volatilized organic pollutants. The liquid products from the
condenser 14 are delivered through a conduit 36 to the collector 17
which is shown as a decanter. Uncondensed organic pollutants are
removed from the decanter 17 through conduit means 40. Light
organic pollutants are removed from the decanter 17 through conduit
means 39. Water is removed from the decanter 17 through conduit
means 38. Heavy tars, asphalts, carry-over particles, etc., are
withdrawn from the decanter 17 through conduit means 37.
The mass flow rate of treatment gas through the conduit means 29 is
from 2.5 to 100 times (preferably 5 to 30 times) the mass flow rate
of treatment gas recovered through the conduits 30 or 31. The need
for substantial volumes of flow arises from the use of the
superheated steam in the treatment gas as a source of heat for
vaporization of volatilizable organic pollutants and for vaporizing
moisture from the contaminated soil within the rotary drum 11. In
some embodiments it may be desirable to provide extrinsic heat to
the exit gas stream in the conduit means 27 as shown schematically
by the heater element 41. The supplemental heat is primarily
intended to preclude condensation of volatilized organic pollutants
within the conduit 27 and the pump means 12 and the conduits 28,
30, 31 and 32.
Solids Removal
It may be desirable to remove gas-borne solid particles from the
recirculating treatment gas in the conduit 27. An appropriate
solids/gas separator 42 may be provided along with conduit means 43
for receiving treatment gas from the conduit 27 and conduit means
44 to return to the conduit 27 treatment gas from which solids have
been removed. The particles from the conduit 45 may be returned to
the rotary drum 11 or may be otherwise managed. It is desirable
that the solids/gas separator 42 be maintained at a temperature
above the condensation temperature of the volatilized organic
pollutants. Appropriate heating means indicated schematically by
the numeral 46, are provided to maintain the solids/gas separator
42 at an appropriate elevated temperature.
EXAMPLES
To illustrate the cost effectiveness of the present invention, four
examples are provided for comparison in which the energy
requirements for decontamination of soils by treatment with
superheated steam is calculated for a process according to the
invention, in which superheated steam is recycled, and according to
a process in which superheated steam is employed without recycling.
In all instances, the calculations are based on the following: (a)
the contaminated soil contains 1% by weight of volatilizable
organic pollutants and 20% by weight moisture at ambient
temperature (60.degree. F.); (b) the decontaminated soil contains
less than 0.5 wt percent water and less than 0.001 wt percent of
residual organic pollutant, i.e., more than 99.9 wt percent of the
volatilizable organic pollutants were removed; (c) the rotary drum
is 5 feet in diameter, weight 7000 lbs. and the soil is heated to
700.degree. F.
Four examples will highlight the benefits resulting from the
practice of the present invention.
In each example, calculations are made based on the system
illustrated in FIG. 1 with certain changes to be described.
EXAMPLES I, II and III illustrate the cost effectiveness of the
invention. EXAMPLE IV is a comparative example and illustrates the
cost of using superheated steam in a process without recycle or
recirculation of superheated steam.
In EXAMPLE I, calculations were based on the system illustrated in
FIG. 1 operated on 1 ton per hour of contaminated soil with
superheated steam being employed to supply the entire heat
requirements of the system.
In EXAMPLE II, calculations were based on the system illustrated in
FIG. 1 operated on 1 ton per hour of contaminated soil with
indirect heat supplied to the rotary drum and superheated steam
employed to heat the soil and to maintain the contaminated soil at
the desired temperature, 700.degree. F., and to offset heat losses
from the system to the environment.
In EXAMPLE III, the calculations were based on a system similar to
that described in EXAMPLE II except that the throughput is 2 tons
per hour.
In comparative EXAMPLE IV, the calculations were based on a system
wherein the rotary drum 11 and the recovery system illustrated in
FIG. 1 were employed, without pump 12, i.e. a system which has no
recirculating superheated steam and in which superheated steam
provides all of the heat energy, is employed on a once-through
basis, and the throughput is 1 ton per hour.
TABLE I sets forth the parameters of each EXAMPLE and the heat
requirements and steam requirements.
TABLE I
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ENERGY REQUIREMENTS FOR SOIL DECONTAMINATION EXAMPLES I II III IV
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FEED SOIL (comparative) Water, wt %* 20 20 20 20 Organics, wt %* 1
1 1 1 Temperature (.degree.F.) 60 60 60 60 FLOW RATE lbs/hr 2000
2000 4000 2000 PRODUCT SOIL Water, wt %** <0.5 <0.5 <0.5
<0.5 Organics, wt %** <0.001 <0.001 <0.001 <0.001
Exit Temperature (.degree.F.) 700 700 700 700 HEAT REQUIREMENTS
(75%; Efficiency), MBTU/hr Heat Soil to 700 (.degree.F.) 520 520
1040 520 Heat, Vaporize Moisture 665 665 1330 665 Heat, Vaporize
Organics 15 15 30 15 Heat Loss 300 300 300 300 TOTAL HEAT
REQUIREMENTS 1500 1500 2700 1500 STEAM REQUIREMENTS S/H Steam
lbs/hr 3925 2250 2250 3925 S/H Steam to Compressor 1770 1015 1015
N/A 220 (.degree.F.) - ACFM S/H Steam to Superheater 1310 750 750
N/A 300 (.degree.F.) - ACFM S/H Steam to Rotary Drum 2820 1600 1600
3925 1000 (.degree.F.) - ACFM INDIRECT HEAT - MBTU/hr 0 635 1835 0
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*Wet Basis **Dry Basis
From TABLE I it will be observed that the steam requirement for
EXAMPLE I is the same as that for EXAMPLE IV. In both EXAMPLES I
and IV all of the heat requirement is supplied by superheated
steam. Similarly, the superheated steam requirement for EXAMPLE II
is the same as that of EXAMPLE III, despite the fact that EXAMPLE
III treats twice the quantity of contaminated soil of EXAMPLE
II.
TABLE II sets forth the cooling requirements for volatilized
organic pollutants and the carrying steam and also a summary of the
overall energy requirements.
TABLE II ______________________________________ ENERGY REQUIREMENTS
FOR ORGANIC RECOVERY EXAMPLES I II III IV
______________________________________ GAS EXIT STREAM (lbs/hr)
(comparative) Steam 3925 2250 2250 3925 Soil Moisture 390 390 780
390 Organics 20 20 40 20 GAS STREAM TO CONDENSER (lbs/hr) Vaporized
Moisture >390 >390 >780 >390 Organics 20 20 40 20
Recirculating S/H Steam 0 0 0 3925 TOTAL LBS/HR TO >410 >410
>820 >4335 CONDENSER COOLING REQUIREMENTS to 60 (.degree.F.),
75% Efficiency 617 617 1234 5000 MBTU/hr ENERGY REQUIREMENTS
(MBTU/ton) S/H Steam 1500 865 433 5300 Cooling 620 620 620 5120
Indirect Heat To Rotary Drum 0 635 917 0 TOTAL ENERGY 2120 2120
1970 10420 REQUIREMENTS MBTU/Ton
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In TABLE II the cooling requirements per ton of soil processed are
identical for EXAMPLE I, EXAMPLE II and EXAMPLE III.
The cooling requirements for EXAMPLE IV are disproportionate
because the superheated steam is employed on a once-through
basis.
Summary of Examples I to IV
From TABLE II it will be observed that substantial savings in the
total energy requirements per ton for the described soil
decontamination are obtained according to the invention (compare
Examples I, II and III with example IV). It will also be observed
that the total energy requirements per ton are least for EXAMPLE
III since the same rotary drum is processing twice the throughput
with the same heat loss. The energy requirements for the method
illustrated by EXAMPLE IV (without recirculation or recycle of
superheated steam) are excessive.
Some of the energy supplied for EXAMPLES II and III is in the form
of burning fuel gas or fuel oil which indirectly heat the rotary
drum and provides thermal energy at relatively low cost and at a
significantly lower cost than electrically heated superheated
steam. It is thus possible to reduce the amount of superheated
steam required to be in the recirculating loop (compare EXAMPLES
II, III versus EXAMPLES I, IV) and to thus reduce the size of the
superheater 13 and pump 12 by the use of supplemental heating
means.
The process of the invention has been used on a bench scale and
found to be effective for decontamination of contaminated soil from
a variety of contaminated sites including a former wood treating
facility that used creosote and copper, chromium and arsenic
formulations and a site contaminated with pesticides.
The method and system of the invention have been found to be quite
flexible with a wide variety of operating temperatures and
residence times depending on the material being treated, are
applicable to the cleanup of a variety of contaminants, and are a
viable alternative for on-site treatment of soils from various
contaminated sites.
The method and system may be used in conjunction with the
remediation of contaminated solid materials as described and
claimed in copending U.S. applications Ser. No. 055,428 entitled
METHOD FOR TREATMENT OF CONTAMINATED MATERIALS WITH SUPERHEATED
STEAM THERMAL DESORPTION AND RECYCLE and U.S. Ser. No. 055,432,
entitled METHOD FOR TREATMENT OF IMPOUNDED SLUDGES, SOILS AND OTHER
CONTAMINATED SOLID MATERIALS, both filed concurrently and commonly
assigned herewith, the disclosures of which are incorporated herein
by this reference.
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